The present invention concerns an electroluminescent lighting device for a dial, and more particularly for a circular dial of an apparatus of small dimensions with an autonomous power source, such as a timepiece to allow the time symbols situated at the periphery of the dial to be read at night.
The use of electroluminescent panel or lamp for lighting a dial at night is well known. In U.S. Pat. No. 4,775,964, an electroluminescent sheet is cut to the dimensions of the dial of a wristwatch, pierced with a hole at its centre for the passage of the shaft of the hands, provided with time symbols by printing or screen printing on the face turned towards the observer and connected, by flexible connectors bonded to each electrode of the electroluminescent sheet, to an electronic circuit provided to supply an appropriated pulsed voltage. The improvements described in U.S. Pat. No. 5,265,071 essentially concern simplification of the connection means which are then formed by a tongue, which may or may not be bent, and of which an exposed portion of electrode is coated with a conductive ink to avoid or compensate for any cracks in the electrode in order to establish at least one contact with the printed circuit. Such a construction offers the advantage of being relatively simple to implement. However it has the drawback of still having a high power consumption, because of the size of the surface covered, despite the progress made as regards the design of the electroluminescent films and excitation circuits, and of restricting the possible aesthetic variations in the dial, which are very often desired by designers to differentiate their products from those of competitors.
In order to reduce power consumption, it is of course possible to reduce the surface of the electroluminescent sheet for example by limiting the surface thereof around the time symbols, as described in German Patent No. 4 023 693, or conversely, by cutting openings in its surface representing said time symbols, as described in Japanese Patent No. A-02-177290, which complicates the mounting of the electroluminescent elements on the dial. Moreover, in order to have a certain aesthetic appearance, the use of several electroluminescent sheets of different colours has also been proposed, for example cut into concentric sectors or rings, as described in Japanese Patent No. A-05-182761. Here again, the design and particularly the connection, becomes complicated and the aesthetic variations remain limited.
In fact, in order to have the greatest possible freedom as regards decoration of the dial, both for observation during the day and at night, it is preferable to have indirect lighting from the flange.
Numerous devices using one or more diodes, which may or may not be associated with a light guide, have been proposed. By way of example, U.S. Pat. Nos. 4,705,407 and 4,908,739 and European Patent No. 0 860 755 may be cited, which disclose various forms or structures of rigid annular guides made of PMMA intended to concentrate the light as much as possible on the dial and to provide uniform lighting. Use has however shown that it is very difficult, even impossible, to have truly uniform lighting.
The object of the invention is to overcome the drawbacks of the aforecited prior art by providing a lighting device using an electroluminescent lamp in the shape of a strip which is easy to set in place and connect to the excitation circuit.
The invention therefore concerns a peripheral lighting devices for the scale of a dial of an apparatus of small dimensions including a case in which are housed a movement driving at least one hand with respect to the scale, a power source and an electronic circuit able to supply a pulsed voltage, said case being closed by crystal extended on the dial side by an annular extension forming a flange, characterised in that a circular recess is provided inside the base of the flange to accommodate on the one hand, an electroluminescent ring formed of a strip having a border extending beyond the base and a connecting lug, and on the other hand a ring made of translucent plastic material forced into the annular recess to hold the electroluminescent ring at the bottom thereof.